Posted 2016-January-15, 12:05
If you're playing "if the parity of the lowest cards in dummy's suits is odd, we play right-side-up, else upside-down", and you get asked this at trick 6 - and that's your answer - I think you're deliberately playing something primarily designed to hide your system (*). I would expect in the ACBL it would be Pre-Alertable (note that the list of things in the Alert Procedure is a "includes" list, intentionally not complete, and this is sufficiently a "system[] fundamentally unfamiliar to the opponents" that even though it's carding, it should apply).
I would suggest that the only reason to play this is to hide your carding; and that while it's not encrypted by the definition, if you use a key that is incredibly likely to be treated as unimportant and don't warn in advance, it's still improper. I would say the same thing of something difficult or impossible for declarer to work out, but still "available to all"; such as "if the sum of the digits of P^board# mod Q is odd, we play right-side-up, else upside-down, for P,Q = [large pseudoprimes that change every tournament]).
Again, the people that play "different" or "difficult to analyze unless you're used to it" systems (bidding or carding) that do it for the benefit it gets are very good at "doing the work" for the opponents. The people that play them to confuse the opponents - don't. The people who say "I can't remember" when in fact what they mean is "I don't have to tell you" - well, I'll leave that to your imagination.
(*) I am reminded of the pair that - pre-Announcements - played 1NT 12-14 VUL/15-17 NV. Oddly enough, when Announcements came in, they switched to the Normal Way. Similarly the pair that played 12-14 NT *overcalls*, until shortly after they were made aware of their requirement to Alert it (although, in that case, they weren't trying anything on. They just didn't realize how much of their "we don't get bad results" came from the opponents' lack of knowledge; until that went away.)
When I go to sea, don't fear for me, Fear For The Storm -- Birdie and the Swansong (tSCoSI)